What is a NaN?!? Fail in my datalogs...

Just finished a new build with some parts I had on the shelf. It's an APM 2.8 with an M8N GPS/compass module. I did a quick test flight and it worked well in Stabilize mode, but didn't try Loiter. When I reviewed the logs I got this error: "FAIL - Found NaN in IMU.GyrZ". I've searched all through the Internet and found nothing related to drones and NaN. Can anyone help explain this to me?